In Releases 8-11 of 3GPP (Third Generation Partnership Project), techniques for forming beams in the horizontal direction at a base station having multiple horizontally arranged antenna ports are employed. Also in Release 12, in order to further improve system characteristics, beam forming techniques (3D MIMO (Multiple-Input Multiple-Output)) for forming beams not only in the horizontal direction but also in the vertical direction at a base station having multiple antenna ports arranged two-dimensionally in the horizontal and vertical directions are being discussed.
In the 3D MIMO, a base station uses multiple antenna ports arranged two-dimensionally in the horizontal and vertical directions to form directional beams in the horizontal and vertical directions. As illustrated in FIG. 1, the base station transmits directional beams, that is, precoded signals from the respective antenna ports. In the illustrated example, the base station transmits precoded CSI-RSs (Channel State Information-Reference Signals) for reception quality measurement at user equipment as the precoded signals. The user equipment measures reception quality of the precoded CSI-RSs transmitted from the respective antenna ports, selects a directional beam having good reception quality (for example, a directional beam having the maximum reception power) based on the measured reception quality and feeds a beam index of the selected directional beam back to the base station.
As implementations of the 3D MIMO, vertical beam forming and FD (Full Dimension)-MIMO are known. In 3GPP specifications, the case where the number of transmission antenna ports is smaller than or equal to 8 is referred to as the vertical beam forming, and the case where the number of transmission antenna ports is greater than 8 (such as 16, 32 and 64) is referred to as the FD-MIMO. In the future, it is assumed that more than hundreds to ten thousands of antennas may be used (Massive MIMO or Higher-order MIMO), and it may be considered that antennas are arranged in three arbitrary directions in addition to the two-dimensional arrangement of the horizontal and vertical directions.
See 3GPP TS 36.211 V12.0.0 (2013-12) for details of the 3D MIMO, for example.